Chloroplasts of C3, C4, and CAM plants have been isolated in isotonic medium without adding artificial reagents. All but those of CAM plants were capable of reducing electron acceptor DCIP. The rate of DCIP photoreduction at light saturation are 300 for spinach mesophyll chloroplasts, 100 for corn mesophyll chloroplasts and 50 for bundle sheath(BS) chloroplasts. These results show that PSII system in BS chloroplast of C4 plants are functioning. The light curves of DCIP reduction for intact spinach mesophyll chloroplasts saturated at 4000 ft-candles(ftc) while the broken one at 2000 ftc. Experiment of ferricyanide reduction showed similar trend. These results suggest: (1) sites of DCIP and ferricynide reduction are same in photosynthetic electron transport; (2) some unknown dark reaction in PSII is rate-limiting. The DCIP photoreduction by intact and broken corn mesophyll chloroplasts saturated at 2000 ftc. In same reaction did not saturate until 8000 ftc for intact BS chloroplasts and 15000 ftc for broken chloroplasts. These suggest: (1) electron transport system between two chloroplast types are slightly different; (2) size of PSI activity in BS chloroplast is far greater than that in mesophyll chroroplast. The saturation at much higher light intensity by BS chloroplasts may rationalize the high efficiency of C4 plant in harvesting the solar energy.